As is known, audio amplifiers of the type referred to above output respective push-pull signals on the output pair having the same amplitude and being adapted to supply a load with a maximum voltage which is twice as high as the power supply voltage.
Such amplifiers currently find their most extensive application to high output power car radio sets.
Low output power car radio sets still make wide use of conventional so-called single-ended stereophonic amplifiers the maximum output dynamic range whereof is the same as the power supply voltage. However, car radio set manufacturers have a need for high power audio amplifiers of the so-called bridge type, for operation as stereophonic amplifiers as well.
The current state of the art provides several solutions to the problem of forming a bridge-type amplifier from a pair of operational amplifiers.
A first solution consists of associating an amplifier, having a single input and a pair of outputs in push-pull relationship, with a pair of power amplifiers having their non-inverting inputs respectively connected to either output. The outputs of such amplifier pair are connected to the load.
However, this prior solution has the drawback that it involves circuit complexity and that the resulting amplifier cannot be used as a stereophonic amplifier.
A second solution consists of associating an inverting amplifier with a non-inverting one, with shared inputs and each output connected to a load comprising a stereophonic loudspeaker. This solution requires, however, a large number of external connecting components, in particular capacitors, and either has a low input impedance, or if the input impedance is high, is affected by high noise in the bridge configuration.
A third known solution comprises two signal amplifiers having high impedance differential inputs connected to the outputs of respective power amplifiers. By connecting the in-phase inputs of the signal amplifiers together and associating the outputs of the power amplifiers with a respective load, the stereophonic configuration is obtained for the whole amplifier.
However, this solution also has a highly complex circuitry and, in addition, requires a large number of pins on the so-called carrier package for the integrated circuit forming the amplifier, as well as a correspondingly high number of external connection capacitors.
Modern car radio sets are highly compact in size and usually comprise four amplification channels for driving four loudspeakers independently, and accordingly, a large number of pins on the respective packages for the various amplifiers are inconvenient to install in car radio set cases for which packages of limited width are to be preferred.
A fourth solution consists of arranging an operational amplifier to drive the output of a second amplifier in an inverting configuration and having unity gain. However, this solution also requires a large number of pins and of external connection capacitors.